Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC{sup +} and HBC{sup 2+}
- Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden (Netherlands)
We present the first infrared (IR) gas-phase spectrum of a large and astronomically relevant polycyclic aromatic hydrocarbon (PAH) cation (C{sub 42}H{sub 18} {sup +}, HBC{sup +}) and its dication (C{sub 42}H{sub 18} {sup 2+}, HBC{sup 2+}). The spectra are recorded via infrared multiphoton dissociation (IRMPD) spectroscopy of ions stored in a quadrupole ion trap, using the intense IR radiation of a free electron laser in the 530–1800 cm{sup −1} (5.6–18.9 μm) range. HBC{sup +} shows main intense absorption peaks at 762 (13.12), 1060 (9.43), 1192 (8.39), 1280 (7.81), 1379 (7.25), and 1530 (6.54) cm{sup −1} (μm), in good agreement with density functional theory calculations after scaling to take the anharmonicities effect into account. HBC{sup 2+} has its main absorption peaks at 660 (15.15), 766 (13.05), 1054 (9.49), 1176 (8.50), 1290 (7.75), 1370 (7.30) and 1530 (6.54) cm{sup −1} (μm). Given the similarity in the cationic and dicationic spectra, we have not identified an obvious diagnostic signature to the presence of multiply charged PAHs in space. While experimental issues associated with the IRMPD technique preclude a detailed comparison with interstellar spectra, we do note that the strong bands of HBC{sup +} and HBC{sup 2+} at ∼6.5, 7.7, 8.4, and 13.1 μm coincide with prominent aromatic infrared bands (AIBs). HBC has only trio CH groups, and the out-of-plane CH bending mode of both HBC cations is measured at 13.1 μm, squarely in the range predicted by theory and as previously found in studies of small (substituted) PAHs. This study therefore supports the use of AIBs observed in the 11–14 μm range as a diagnostic tool for the edge topology of large PAHs in space.
- OSTI ID:
- 22869424
- Journal Information:
- Astrophysical Journal, Vol. 836, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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